The capacity of implantable cardiac pacemakers to perform multiple functions over the lifespan of a significant number of years has improved dramatically with technological advances. Thus, today's pacemakers often feature the ability to operate in different modes, to adapt pacing rate to sensed physiological parameters, to store significant information regarding pacemaker operation and sensed patient events, to detect and respond to tachycardia and to perform a number of other important and significant functions over and above delivering pace pulses. A modern pacemaker can be programmed by an external programmer, and can both receive information from an external source and transmit collected data to an external source. Most present day pacemakers employ a microprocessor or some sort of microprocessor logic for software control of a number of the pacemaker operating functions. However, despite vast improvements in pacemaker batteries and in miniaturization and efficiency of microprocessors and other components, providing for all of these functions and operations makes a substantial demand on the pacemaker's battery. The lifetime for presently available implantable pacemakers varies within a range of about 5-10 years. For certain smaller pacemakers, the usable lifetime may be in the order of 5-6 years.
Whether or not the pacemaker industry can look forward to improvements in battery design and capacity, there is an ongoing need to improve pacemaker technology to the extent possible to decrease battery drain, and thus maximize the usable lifetime of the pacemaker, whether it is a relatively small or a relatively large design pacemaker. For this reason, there have been a number of improvements designed to minimize current drain of the pacemaker when in operation. Many pacemaker designs incorporate what is referred to as a "sleep mode" feature, whereby the microprocessor is operated at a lower clock frequency during a portion of each pacemaker cycle when it is not actually carrying out instructions. This technique reduces the current drain of the microprocessor for the periods when it is in the sleep mode, or standby mode. Another relatively old technique is to use an external magnetic signal and a reed switch in the implanted pacemaker, arranged to disconnect a circuit from the pacemaker battery when the circuit is not going to be utilized.
The specific problem that this invention addresses, and which presently represents a significant need in the art, is that of increasing the shelf life of the pacemaker, i.e., minimizing the current drain of the pacemaker after its fabrication has been completed and until the time that it is actually implanted in a patient. For a typical current model small pacemaker having a lifetime of 5-6 years, the shelf current drain can be as great as 10 microamps. If the pacemaker spends a year from time of fabrication until it is implanted, i.e., on the shelf, it can discharge approximately 10% of its capacity during that year, thus significantly reducing its usable lifetime. However, if the shelf current drain can be reduced from about 10 microamps to about 2.5-3.5 microamps, it is apparent that the usable operational lifetime of the pacemaker can be increased.
There have been some attempts to avoid current drain prior to actual implantation, and thus minimize battery depletion during shelf life. Thus, Vitatron Medical, B.V., assignee of this invention, had an early 1960's model where the pacemaker was turned on by screwing on the indifferent plate at the time that implantation was anticipated. Other pacemaker models manufactured by Medtronic, Inc., entailed similar type of pacemaker turn-on by use of a "pigtail" connected battery. However, these techniques are not currently useful in view of modern techniques for manufacturing and sealing the pacemaker so as to maintain long life integrity of the pacemaker against ingress of body fluids. Another technique that has been utilized by Medtronic, Inc., in connection with its Activitrax.TM. pacemaker, has been to ship the pacemaker at a relatively low non-rate responsive mode, so that until it is implanted it operates at a reduced rate and relatively low current drain. At the time of implantation, the pacemaker is programmed to the rate responsive mode, where it operates at a relatively higher current drain. While this technique affords a distinct savings in current drain for the duration of the shelf life, it is limited to taking advantage of only the current drain savings that results from operating at a lower base pacing rate. By contrast, this invention seeks to take advantage of all opportunities to reduce current drain during shelf life, and still provide a safe and reliable means for switching from a low power standby mode to a full power operational mode when implantation is imminent.